A poly(para-phenylene) with hydrophobic and hydrophilic dendrons: Prototype of an amphiphilic cylinder with the potential to segregate lengthwise

Article abstract of DOI:10.1002/(SICI)1521-3773(19990816)38:16<2370::AID-ANIE2370>3.0.CO;2-T

Langmuir monolayers are formed from an amphiphilically decorated poly(para-phenylene), which indicates that its hydrophilic and hydrophobic parts segregate lengthwise along the polymer backbone in this nanometer-sized cylinder as illustrated in A. This po

Full text of DOI:10.1002/(SICI)1521-3773(19990816)38:16<2370::AID-ANIE2370>3.0.CO;2-T

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A Poly(para-phenylene) with Hydrophobic and
Hydrophilic Dendrons: Prototype of an
Amphiphilic Cylinder with the Potential to
Segregate Lengthwise**
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Zhishan Bo, Jürgen P. Rabe, and A. Dieter Schlüter*
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Two subclasses of dendrimers[1] have recently been devel-
oped intensively, namely those with on the one hand dotlike
cores and amphiphilic character and on the other polymeric
cores and homophilic character. Interest in the first class
arises mainly from their representativesꢁ unusual size and
shape that complements the array of commonly known much
smaller amphiphiles.[2] The representatives of the second
subclass are interesting because of their unprecedented
mesophase behavior[3] and the finding that some of them
can be considered as cylindrical, shape-persistant nano objects
with lengths in the range of 30 ± 100 nm and diameters of 2 ±
5 nm.[4] In view of these important developments for both
supramolecular and nano chemistry, we decided to merge
these subclasses by providing access to amphiphilic cylinders,
which, depending on the surrounding medium, could segre-
gate lengthwise into two different
a
COOCH3
X
X
1
3
4
3a, 4a : X=CO2CH3
3b, 4b: X=CH2OH
3c, 4c: X=CH2Br
3a
4a
4b
4c
b
e
d
3b
3c
c
f
Scheme 1. Synthesis of the hydrophilic dendron 4c: a) K2CO3, DMF,
908C, 24 h, 68%; b) LiAlH4, THF, RT, 24 h, 86%; c) CBr4, PPh3, THF, RT,
76%; d) methyl-3,5-dihydroxybenzoate, K2CO3, acetone, reflux, 24 h,
99%; e) same as b), 88%; f) same as c), 61%.
the selective oxidation of one methyl group of 5a with nitric
acid[7, 8] to give 5b (Scheme 2). Esterification and bromination
afforded 5d whose conversion into 5e was done with hydro-
quinone in tenfold excess. After purification by filtering
through a short column followed by recrystallization the yield
reached 78%. The coupling of Frechet dendron 6 with 5e
halves (see schematic representa-
tion A). This structural motif is
rather unique. In nature it can be
found in some ion-channel mem-
[9]
Â
brane proteins. The targeted macromolecules, therefore,
attracted our interest as models for such proteins.[5] They
may also serve as novel and giant constituents of self-
aggregated assemblies and should show interesting behavior
at interfaces. Thus, a polymer was required, preferentially of
the rigid-rod type, whose repeating units are equipped with
two sterically demanding substituents, one of which being
hydrophobic and the other hydrophilic. The synthetic strategy
for this target should be flexible in regard to the achievable
hydrophilicity/hydrophobicity ratio, as this ratio is well known
to have a strong impact on the aggregation behavior of
amphiphiles.[6] Here we describe the synthesis of the Suzuki-
type monomer 8 equipped with unlike dendrons and its
polycondensation with diboronic acid 9 to give the prototype
amphiphile 10. The behavior of 8 and 10 at the air/water
interface is also described.
5a: Y=CH3; Z=CH3
a
Y
5b: Y=CO2H; Z=CH3
b
Br
Br
5c: Y=CO2CH3; Z=CH3
Z
c
5d: Y=CO2CH3, Z=CH2Br
5e: Y=CO2CH3; Z=CH2OC6H4OH
5
d
Scheme 2. Synthesis of the unsymmetrically substituted dibromobenzene
5e: a) 45% HNO3, 7 d, 1008C, 92%; b) CH3OH, H2SO4, reflux, 16 h, 87%;
c) CH2Cl2, Br2, 158C, UV, 66%; d) hydroquinone (10 equiv), K2CO3,
acetone, [18]crown-6, reflux, 24 h, 78%.
gave 7a whose ester group was converted into the hydroxy-
phenyloxymethyl substituent in 7d through a short sequence
(Scheme 3).[10] The final attachment of hydrophilic dendron
4c to monodendronized dibromide 7d gave the amphiphilic
monomer 8[11] on the 3 g scale.
The synthesis of 8 and 10 are described in Schemes 1 ± 4.
Dendron 4c was prepared by standard procedures
(Scheme 1). Despite the required purification by repeated
column chromatography it was obtained on the 10 g scale. The
substitution of 8 with unlike dendrons was brought about by
A purity of over 98% was estimated for 8 from its 500 MHz
proton NMR spectrum. This high purity is further backed by
analytical gel permeation chromatography (GPC; UV and
refractive index detection), which shows only one, fully
symmetrical trace with a polydispersity of less than 1.01.
Suzuki polycondensation of 8 with diboronic acid ester 9 [12]
was done under standard conditions[13] in 2n NaHCO3 and
THF with [Pd{P(p-tolyl)3}3] as the catalyst precusor
(Scheme 4). Standard work-up afforded 10[11] as a white
amorphous material in a yield of 95 ± 98% and in quantities
exceeding 1 g. Its molecular weight was determined by GPC
versus a polystyrene standard as Mn ˆ 38000 (Pn ˆ 20), Mw ˆ
85000 (Pw ˆ 45), and Mw/Mn ˆ 2.3. It should be noted that
according to molecular weight determinations of various
[*] Prof. Dr. A. D. Schlüter, Dr. Z. Bo
Institut für Organische Chemie der Freien Universität
Takustrasse 3, D-14195 Berlin (Germany)
Fax: (‡49)30-838-3357
E-mail: adschlue@chemie.fu-berlin.de
Prof. Dr. J. P. Rabe
Institut für Physik der Humboldt Universität
Invalidenstrasse 110, D-10115 Berlin (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(Sfb 448, Teilprojekte A1 and B5) and the Fonds der Chemischen
Industrie. We thank Prof. J. H. Fuhrhop for helpful discussions.
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Angew. Chem. Int. Ed. 1999, 38, No. 16

COMMUNICATIONS
O
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+
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7a: Y=CO2CH3
7b: Y=CH2OH
7c: Y=CH2Br
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7d: Y=CH2OC6H4OH
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Scheme 4. Polycondensation of monomers 8 and 9: a) NaHCO3, THF,
H2O, 1 ± 1.5 mol% [Pd{(P(p-tolyl)3}3], reflux for 3 d.
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7d + 4c
Br
Br
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Figure 1. Surface pressure ± area isotherm of 8 and 10 (X in Š per r.u.;
1 and 2 indicate the first and second compression).
Scheme 3. Synthesis of 8: a) K2CO3, acetone, [18]crown-6, reflux, 24 h,
86%; b) LiBH4, THF, reflux, 8 h, 97%; c) CBr4, PPh3, THF, RT, 42%;
d) hydroquinone (20 equiv), K2CO3, acetone, [18]crown-6, reflux, 24 h,
78%; e) K2CO3, acetone, [18]crown-6, reflux, 24 h, 63%.
larly to the monolayer,[16] thereby defining a minimum area
per molecule. In comparison, 10 exhibits a 10% larger area
per repeat unit (0.82 nm2 per r.u.) upon the first compression
with a hysteresis in the first decompression and a shift to a
more reversible isotherm and a smaller area per r.u. in the
second cycle (0.77 nm2 per repeat unit (r.u.)). The good
agreement between the areas per molecule of 8 and per r.u. of
10 indicates a structure of the polymer monolayer in which the
rodlike polymer molecules are oriented with their long axes
within the monolayer plane and close packed. Moreover, the
hydrophilic ethyleneoxy chains are on that side of the polymer
that faces the water subphase, while the more hydrophobic
dendron faces the air side of the polymer. This picture is
supported by a control experiment on a closely related
other dendronized polymers by small angle neutron scattering
(SANS) GPC tends to underestimate the actual molecular
weights. Typical factors by which molecular weights deter-
mined by GPC versus the polystyrene (PS) standard should be
multiplied to get the actual ones lie in the range of 1.5 ± 4.[14, 15]
Langmuir monolayers of 8 and 10 have been prepared at
the air/water interface. Surface pressure ± area isotherms
(Figure 1) at room temperature reveal stable monolayers
(area change is less than 1% during 20 min) at pressures of
1
more than 20 mNm . The monolayers of 8 exhibit very good
reversibility for compression, decompression, and repeating
cycles, with an area per molecule of about 0.73 nm2 per
1
molecule at 20 mNm . This is consistent with a monolayer
polymer in which the hydrophilic dendron of 10 is replaced by
[17]
Â
structure in which the four ethyleneoxy chains per water
soluble dendron are close packed and oriented perpendicu-
another G2 Frechet dendron. Under the same conditions
this polymer does not form a stable monolayer at the air/water
Angew. Chem. Int. Ed. 1999, 38, No. 16
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13C NMR (125 MHz, CDCl3): d ˆ 58.98, 67.29, 69.11, 69.55, 69.79,
69.92, 70.30, 70.59, 71.79, 100.92, 101.35, 105.90, 106.18, 115.68, 115.71,
120.85, 127.46, 127.91, 128.47, 132.08, 136.59, 137.53, 138.93, 139.08,
139.49, 139.53, 152.35, 153.23, 153.26, 159.91, 159.99; elemental analysis
calcd for C110H116O24Br2: C 66.66, H 5.90; found: C 65.97, H, 5.78. 10:
1H NMR (500 MHz, CDCl3): all signals are broad, d ˆ 3.31 (12H), 3.50
(8H), 3.65 (8H), 3.77 (8H), 4.03 (8H), 4.82 ± 5.01 (24H), 6.40 ± 6.63
(20H), 6.80 ± 6.82 (8H), 7.28 ± 7.38 (20H), 7.53 ± 7.68 (4H); 13C NMR
(125 MHz, CDCl3): d ˆ 59.00, 67.30, 68.55, 69.56, 69.76, 69.89, 70.34,
70.59, 71.81, 100.95, 101.38, 105.95, 106.21, 115.59, 115.77, 127.49, 127.91,
128.48, 129.16, 131.38, 134.17, 136.64, 138.98, 139.14, 139.49, 139.59,
140.76, 152.86, 153.07, 159.89, 159.93, 159.99; elemental analysis calcd
for (C116H120O24)n: C 73.40, H 6.37; found: C 72.49, H 6.28.
[12] U. Lauter, W. H. Meyer, G. Wegner, Macromolecules 1997, 30, 209±2101.
[13] J. Frahn, B. Karakaya, A. Schäfer, A. D. Schlüter, Tetrahedron 1997,
53, 15459 ± 15467.
[14] S. Förster, I. Neubert, A. D. Schlüter, P. Lindner, Macromolecules, in press.
[15] The 500 MHz 1H NMR spectrum of 10 shows some low intensity
signals which we tentatively assign to end groups. The intensity ratio
between the end group signals and that of the main ones implies a
degree of polymerization of 30 < Pn < 40. This range has to be
considered with some care because of partial signal overlap.
[16] A. I. Kitaigovodstry, Mixed Crystals, Springer Series in Solid-State
Sciences 33, Springer, Berlin, 1984, p. 353.
interface.[18] Monolayers of 10 can be transferred on to mica to
give homogenous films of 3.3 nm thickness with a transfer
efficiency greater than 96%.
In conclusion, we have synthesized a new type of Suzuki
monomer that carries G2 hydrophobic and hydrophilic
dendrons and shown that it can be polymerized to give the
first length-wise (not block-wise) amphiphilically equipped
poly(para-phenylene) 10. Polymer 10 differs from known
amphiphiles in that it consists of a linear, covalently bound
sequence of ªlittleº amphiphiles. It differs from common
ªpolysoapsº because it is much more rigid, which should
increase its potential to aggregate, for example, into channels.
Langmuir± Blodgett experiments provide the first evidence
that the dendritic substituents of 10 segregate lengthwise into
hydrophobic and hydrophilic domains (see schematic repre-
sentation A).
Experimental Section
Langmuir monolayers were prepared by spreading 100 mL of solution in
CHCl3 (1 gmL 1) on a distilled water subphase in a Langmuir ± Blodgett
trough (NIMA Ltd.) at room temperature. Compression rates were on the
order of one percent per minute. The stability of the monolayers was
[17] Z. Bo, A. D. Schlüter, unpublished results.
[18] For the behavior of a polythiophene with ethyleneoxy and alkyl chains
at the air/water interface, see T. Bjornholm, D. R. Greve, N. Reitzel, T.
Hassenkam, K. Kjaer, P. B. Howes, N. B. Larsen, J. Bogelund, M.
Jayaraman, P. C. Ewbank, R. D. McCullough, J. Am. Chem. Soc. 1998,
120, 7643 ± 7644.
1
checked by compressing them to 20 mNm and monitoring the area for
20 min.
Received: December 28, 1998
Revised version: April 7, 1999 [Z12828IE]
German version: Angew. Chem. 1999, 111, 2540 ± 2542
Keywords: amphiphiles ´ dendrimers ´ monolayers ´ nano-
structures ´ polymers
The First Efficient Hydroaminomethylation
with Ammonia: With Dual Metal Catalysts and
Two-Phase Catalysis to Primary Amines**
[1] G. R. Newkome, C. N. Moorefield, F. Vögtle, Dendritic Molecules.
Concepts, Syntheses, Perspectives, VCH, Weinheim, 1996.
[2] F. Zeng, S. C. Zimmerman, Chem. Rev. 1997, 97, 1681 ± 1712; A. P. H. J.
Schenning, C. Elissen-Roman, J.-W. Weener, M. W. P. L. Baars, S. J.
van der Gaast, E. W. Meijer, J. Am. Chem. Soc. 1998, 120, 8199 ± 8208,
and references therein.
Burkhard Zimmermann, Jürgen Herwig,* and
Matthias Beller*
[3] V. Percec, C. H. Ahn, G. Ungar, D. J. P. Yeardley, M. Möller, S. S.
Sheiko, Nature 1998, 391, 161 ± 164.
Dedicated to Professor Rüdiger Selke
on the occasion of his 65th birthday
[4] B. Karakaya, W. Claussen, K. Gessler, W. Saenger, A. D. Schlüter, J.
Am. Chem. Soc. 1997, 119, 3296 ± 3301; W. Stocker, B. Karakaya, B. L.
Schürmann, J. P. Rabe, A. D. Schlüter, J. Am. Chem. Soc. 1998, 120,
7691 ± 7695; ªDendrimers with Polymeric Cores: Towards Nanocy-
lindersº: A. D. Schlüter, Top. Curr. Chem. 1998, 197, 165 ± 192; W.
Stocker, B. L. Schürmann, J. P. Rabe, S. Förster, R. Lindner, I.
Neubert, A. D. Schlüter, Adv. Mater. 1998, 10, 793 ± 797. For a recent
work on dendronized poly(phenylenevinylene)s, see Z. Bao, K. R.
Amundson, A. J. Lovinger, Macromolecules 1998, 31, 8647 ± 8649.
[5] N. Voyer, J. Lamothe, Tetrahedron 1995, 51, 9241; N. Voyer, M.
Robitaille, J. Am. Chem. Soc. 1995, 117, 6599 ± 6600.
[6] ªMolecular Assemblies and Membranesº: J.-H. Fuhrhop, J. Köning in
Supramolecular Chemistry (Ed.: J. F. Stoddart), Royal Society of
Chemistry, London, 1994.
[7] W. Marzin, J. Prakt. Chem. 1933, 138, 103 ± 110; L. Lamba, J. S.
Jaydeep, J. Tour, J. Am. Chem. Soc. 1994, 116, 11723 ± 11736.
[8] Traces of the dioxidation product can be removed by column
chromatography. This, however, was not normally done and the raw
material was used in the next step.
Aliphatic amines are amongst the most important bulk and
fine chemicals in the chemical and pharmaceutical indus-
tries.[1] Alongside hydroamination,[2] hydroaminomethylation
of olefins to amines represents an atom-economic efficient
[*] Dr. J. Herwig
Celanese GmbH ± Werk Ruhrchemie
D-46128 Oberhausen (Germany)
Prof. Dr. M. Beller
Institut für Organische Katalyseforschung
an der Universität Rostock e.V.
Buchbinderstrasse 5 ± 6, D-18055 Rostock (Germany)
Fax: (‡49)381-466-9324
E-mail: matthias.beller@ifok.uni-rostock.de
Dipl.-Chem. B. Zimmermann
Technische Universität München
Lichtenbergstrasse 4, D-85747 Garching (Germany)
Â
[9] J. M. J. Frechet, K. L. Wooley, C. J. Hawker, J. Am. Chem. Soc. 1991,
113, 4252 ± 4261.
[10] It should be noted here that the initial reduction of 7a could not be
done with LiAlH4 because this leads to undesired debromination.
[11] 8: 1H NMR (500 MHz, CDCl3): d ˆ 3.38 (12H, s), 3.67 (8H, t), 3.71
(8H, t), 3.83 (8H, t), 4.13 (8H, t), 4.94 ± 5.01 (24H, seven partially
overlapping s), 6.45 ± 6.67 (18H, eight partially overlapping s), 6.91
(8H, narrow AA'BB'), 7.31 ± 7.47 (20H, m), 7.77 (1H, s), 7.78 (1H, s);
[**] We thank Celanese GmbH for supplies of TPPTS und NAPHOS; Dr.
R. Fischer (Celanese GmbH), Dr. H. Geissler (Clariant AG), and
Prof. Dr. K. Kühlein (Hoechst AG) for support of the project; and
BMBF for financial support. We also thank Prof. Dr. O. Nuyken (TU
München) for the availability of laboratory space and additional
infrastructure.
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Angew. Chem. Int. Ed. 1999, 38, No. 16